The "AZW3" extension can always be replaced by "Mobi". I don't know if that's true for AWZ2 or not.

Changing the extension is poor practice just to get it to upload. I would suggest putting it in a zip file. Yes, for Kindle devices you could get this to work but many users download files in mobi formatting for devices other than a Kindle so this could be confusing when it did not work on their devices.

Of course, MR users could either rename .azw3 files or zip them up, but it'd much easier if someone simply added azw3 to the list of allowed extensions in the PHP code, which is a trivial change that'd take a couple of minutes at most.